Answer: B – Smallpox
The germs of smallpox paved the way for Europe's conquest of America. Native Americans had no immunity against it; it spread rapidly, killing entire communities, making conquest easier for the Spanish.
Source: Chapter 3, Section 1.3 – Conquest, Disease and Trade
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The passage explicitly states: "the most powerful weapon of the Spanish conquerors was not a conventional military weapon at all. It was the germs such as those of smallpox." Smallpox killed whole communities before Europeans even arrived, directly enabling conquest. Examiners expect the correct option (B) and may award the mark only for that — no partial marks in MCQs.